Intermodulation meter



J. 1.. HATHAWAY 2,520,766

INTERMODULATION METER 2 Sheets-Sheet l m MT Aug. 29, 1950 Filed June 18, 1947 m M/ m z A. Q T m W g v W I J T N EQ Q 3 m m m Q n H w hr Q @Q T x Aug. 29; 1950 .1. HATHAWAY 2,520,766

INTERMODULATION METER Filed June 18, 1947 2 Sheets-Sheet 2 ATTORN EY INVENTOR.

1 corded program. sounde'di distorted;

Patented Aug. 29, 1950 it wrmnmoiiiiilin tion'of Delaware--- Application-lumen; 1947, seniar nor-{sauna IB CIaTmsT 01. 179-171 Anotherobject-1o: -theepresent. invention is .the-

provision of an- :intermoduletion. meter as arc-re. said which less and complicated than metersheretoiore. known. Anotherobject oi thepresentinvention ptovision of an intermodulation .meterrwhich. ops erates'on. :a. single. tone frequency. combination. While. {previously knownintermodulation me ters .utili'zinm-severak tone irequencypcombina tions. arev more versatile: than the present rangement,,1 have discovered that a singiettonew frequency ins-an intermodulation meter givvesoae satisfactory, tester-audible distention. Earthen more,;,.the use one singlertone frequencycomiob nation simplifies the structure of theintermodu-e Lationv meter considerably I and makes. :it. "lighter; morecompact andrmoreeasily transportable; In tests which ".I L have made or recordings with; a. single intermodulati'on tone rombination, I have discovered that when the-intermoduIation oi single, pair .of...tones measures low w-iththestruc ture of ..'the present ,invention the recorded .pro. gram. sounded .d'istortion-..free; Conversely, when the intermodulationwas .high as measured; with. the -single tone frequency combination the re-' Itis .thus. felt that the present. intermodulation method'ofi distortion measurement in amplifiers andjinrecording systems is superior to'heretofore known. systems and is more easily accomplished.'

When two tones offdflferent frequency am.

combined in a nonlinear device or in a device.

which distorts these frequencies, j'harmonics or each are xproducedfi. Also; :sum and?cii'fle'rencc tones result; termed cross-modulation. With. the present tones o1400'and 4000"cyc1es per. second are transmittedlinto the device' under? test-and certain Tones, only, .ofthespuri'ons distor; tion" tones measured; The most important harmonic and spurious tonesencounteredare -tfiose ofthe "following. frequencies: 800 11200, 33200,

3600, 4400,480Q 8000 and 1Zlltltiilcycles..pser"secand. Now, 112111.101 these harmonic .and.,snur1ous l tones. are to-be measured to .obtfiinan indication. otthe. distortion. produced, the.--system...undr.

measurement "must. flat up; to at Least-12mm cycles. per second; .Howerer, the present inter: modulation system. meesnresthe sunrandgdifim ence .frequenciesoofithe. 400. cycles per. second; and .4900; cycles persecond t0nes..:which1 are, loa catedjon each side oL-theAoQoIcycles perseccnd! tone; that. is, the tones of frequencies of 13209; 3600544110. 'andf4800 'bycle r second; flihe.,.pr.ee-I

ent arrangement.comparessthe, totaiamplitude.

otithese ton'estmthatof'the .4Q0.0;cyc1es sec-:

and. tone. It; willllbenotedgthat 'for. thisztypeinty measurementxia. fiat response over a wide ire.

quencyjbiandfi's' .unnecessary,, tact 'yery.little dfiierence is noted. even with fairly; Immune,

partures fromratfiat response...

The present arrangement includesv wave. teens erating means operating to generate tone. fr es quencies of 400.;eyc1'es :per second and eonocyglesarranged. .to strongly. discriminate. againstl the.

iooflcycIes per. second tone but to-zpass. the major.

mtermodulation products;; tha 3200. y 36 9.;

4400, and 4810.0 .cycIes per second The combined.

amplitude. of "the .mtermodulation products; then measured.'., The ratio of the amplitude of.

the intermoduiatfon products and the AQQQicycIes per second. tone plus "the intermodulation prod cats 'i a. measure. Jot the amountofi distortion caused iby, the device, under test,

The presentlinvention will he more. .tuillyguna.

derstood. by reference to. the renewing 1 detains.

Figure? is a curvesnqwm cheactmn more selective .nett orksliinao portion. oi the mentzotiFfgure 12.

Referring now to Figure I; the double .triod j;

9 ihassitsI..separate..sections coupled torresonant circuits to form .aAODO cycle tone. generator .111 andraAOO cycle. tone generator ll. Thus the tr'iode Us coupled to atankcircuitincludin in: euctancelzlendcapacity. l'ttunert-totiflmeycles undertest, these. mrth'cr..seiectivenetworks being...

as well as for the remaining tubes in the device is supplied by a conventional power supply including transformer ll, full wave rectifier 152 and a filter arrangement including condenser I53 and series resistance I54. Theoutputfromthe 4000 and 400 cycle tone generators l0 and 'H is taken from taps on inductances i2 and 22 through resistors l6 and l? to the opposite ends of variable voltage divider resistor 18. The control grid 24 of a triode'amp1ifiern26 is connected to the ar ble-tap onresistor I Thus. wh th va ab ap is placed at the' top end of resistor l8 si bstantially all of the input tocontrolj grid 24' comes from the tone generator lil, while when the variable tap is at the bottom o f"res'ist or I8 siibstantially all of the input which is' applied to control grid 24 comes from; the tone generator 1] In order to minimize I distortion a certain amount of] negative feedback ma e applied to the amplifier including tube Z6 by means of resistor 21; g

" tube 90.

and condenser 28"connected from anode 29 to grid '24; The anode 29. isconnected through the primary 30 of the outputjjtransformer 3! to the source'of anode potential "(anode supply l5. v The.

secondary winding "32 of the output transformer 3t' isco'rinected to terminalsfiito whichth e input of the device under test is to be connected. Across the anode circuito f tube'ZS'iS connected a voltagedivider includingserially connected renea est, 35 and 3'5. ThefiVU meter 31 is ar' ranged 'to'be connectedacross selected portions of the voltage divider constituted by resistors 33, 35.

and 36 by'm'eans of the 'selec'tive action of switch 41;." Switches iii and 42; asjindicatedf by the dotted line, are ganged togetherfor simultaneous operation." Thus,jin positions 1,"2'and' 36f the switchgang thefVU meter'31 is arranged'tobe selectively connected across portions of the voltage divider gang 32,135 and 36, "Switch 300f the switch gang in position I disablesthe tone genera ori' fl. 1 At the same timeVUmeter 31 is con nectediacross resistor 36 so that its reading'isl related to the total "signal amplitude across the anode circuit of the tube '29'as the Value of re; sistortfi is related. to the' total resistance of 34; 3 5and36. Therefore"; the VU meter gives a read-v ing proportional to the amount'of tone generated by a tonegenerator H appearing. in the'anode circuit of amplifier tube 25. 'In' position} of the switch gang theoutput circuit of the tone generator H is disabled;- The W meter .31 is connected across Stand 36 andthe VU, meterreading is related to the' total amplitude of the tone generated by'tone generator I0 appearingin the anode circuitof amplifier tube 26 as the value of the resistors 35 and 3,6 is relatedtofthe', total resistance of, 34; 35' and36'. Resistors 35 and 36 are so" proportioned; that equal readings areob itamed on the vumeter'si' for a.4:, 1 ratioofthi 4000 cycle tone to the/100 cycle' tone'." The VU meter 31 iscoupled to primary sideof transformerf3l rather than to the output winding 32 bee cl'ausethe meterre'quires higher voltage for its operation than isavailable across secondary Winding 32. However, the voltagesare in direct proportion so the'operatio'n of'the system is not affected by this connection. The output of the system under test is coupled to terminals 56 which are connected through resistors El and condenser 52 to the primary 53 of an input transformer 5 Condenser 52 is so related to the inductance of the primary winding 53 as to discriminate against frequencies much below 4000 cycles. (londenser 'fif-i coupling 1th secondary 5540f the input transformer to the control grid 58' of the input tube 66 is also so dimensioned as to discriminate against frequencies much lower than 4000"c'ycles. The relative amplitude of signals applied to control grid 58 may be adjusted by .suitablypositioning the movable tap on input potentiometer 61. Across the output of tube 60 is connected a voltage divider including serially connected resistors- 53 and 65. This divider is proportioned to afiord the desired comparison of intermodulation products to 4000 cycle per secnd toneplus. .int d ro u s.- aw ne'ction' from the junction of the resistors goe's'to position 40f switch ng} The arm of switch" 4211 connected to the gridgof amplifier'tube F9. The output circuit of tube to .is connected to tap s;4 and Eof sWitch ll lThus whenfswitch" li'fis in position 4 or' 5, theVU'ineter'3'i is, connected to; the output Ofamplifier tube ltfwhil'e the input (if-tube 19 may be selectively connected to the junction of resistors 63 and 649i" to'the output'ot Connected to the output off amplifier tube Q0 is a er ene al ii u i e, i m lify atq es; f 8t and 953 and associated tuned circuits "8 i582 ss aa; 'aisa;-9'| -s2 and as 9 5.f. In the grid circuit of tube-8t connected as a-Tnetwork is a2 tuned circuit including inductance 8 I and con-* denser 182 sodir'nerijsionia as to discriminate against frequenciesof the order of 4000 cycles.- The plate circuito'f tube 'includes'a tunedciiu cuitincluding inductance- 83 and condenser Bil broadly tuned in the vicinity of 4900 cycles per second. Across tuned circuit 83 -84 is connected the input of a further amplifier tube's 'whichf has 'c'oupled in its plate circuit a further tuned circuit including inductancef'lil and capacity 88; of characteristics similar "to 83- 84." Between tubes 86' and90 is coupled a"further Tnetwork including inductance; 9i and capacity 92 across the top of the T. This network is' seame sioned as to discriminate againstlf requencies of the order. of 3960 cycles iper' second.v The 1 out? put of amplifiertubeSiElf-is coupledflto al final T networlrincluding across'the top'ofthe Ta shunt: tuned circuit including 'an meeetane se and canacity 5 S0 d m n ei e re; ie t fequen l of the order of 4040 cycles'per second.

The overall. efiect :of the chain fof filterj net= worlgsfand peaking circuitsds' to broadly accept frequencies within a fairlybroadband centered about 4000 cycles .p'e'r second je'xcept iorireouencies of 3940;.4000 and 4960 cyclesi per"secondi The overall frequency response of the network between tubeytti and tube 70 is-shownin Figure 2. It will befs'een that for all ffreiuencies betw 1 3 930 and 4070 cycles a per second the. rej ection 'isjin excess-of 50'db. while ior'freqliencies of the or der, qr 3200, 3600, 4400 and44800 cyclesper second as. indicated by'the dots on the .curve,.me, .respo s is down lessthan 3 dbfFdif.frequenciesconsider-.1 abiy' remote from .4000 cyclesI' Qthe response; is down20 db orbetterf I H 1.. I

The device described a n. t l w n man e meter tone output may be tem under anaemia:

.m., overall recording systein ls 'to' 'be "measured ahead -of 1 this in the recording ehannel. tfl he tone may recorded and latw played back intotthe intermodulationmeter:by connecting'the mitput -of ithe pl-ay baek amplifier toiterminalst'ifl It -willethus be seen that the present-arrangemerit perm'its measuring aaudible zdistortion 1-ineluding al'l amplifiers nsed -in recordingiand playback as well as that occurring z inrthe recording cutter, the :pick-np and the record itself. .If an individual amplifier :is xto 'be checked for inter:- modmation, the :terminals 313 areeonneeted :to .ttiez innut.-.ofithe amplifier,.-and.thegamplifier out- .mittiproperlyz terminatedi'i returnedi'tto :terminals .Sllofithe intermodulatiommeter. .flhe-tone out-putrfed; intothexsystem orsamplifier underztest :may :bezadjllsted ;.by throwing the multiple ganged :swi-tehes 40,24! and .42 oalter- 'mtely between positions .1 and 2,. .Themotentiemeter alt: israidj us'tednozthat the reading of VU meter :31 .z-remains constant when the switch is .thmwnzfrom one nositionto .thezother. ,lteshould be noted :thatatherexaetavalueyof thereadingpf :3 11s znotgimpontant z but the reading in'the two-positions of the switch should be approximatel equal. .The ganged switches :may thenrbe thrown toany. one .of the three remaining ;-positions and ;the .intermodulation tones of must relative vamplitude balance will Joe transmitted 111D terminals 33 ata-levelwo the order-of izliheaeorreot intermodulation tone level to be ggplied'to the input of the, system under testis adjusted :by means -of ;potentiometer 51 .making 'forereeommended that insteadotlZdhabove program level, the ,tonelevel should be setahout thegsameas programlevel.

lJntermodulation.readings maynow .be made by switching ganged switches 40, AI and llzifrom step 4, ..a,r ef erence step, toisteplh and' equalizing the readings ofmeter .31 .by adjusting the potentiometer 160. The positionof the control of potentiometer .latfisa measure of the amountiof intermodiilation;producediin the apparatus under test. It will ibe noted that when the ganged switches 1410 M and '42 are onpos'ition l, the VU meter .reads as .a function of the total amplitude OI signals applied 'to inputterminaIs'SlB whilein position}? themeter readsthe input level toter minalsifl. minus the frequencies which have'been eliminated "by the filter shown between tube ice and).

."The' potentiometer 'I'Bll may be directly calibrated' in percentages "by'tlie followin procedure. The input potentiometer 6! is set at any arbitrary value and "a 4000 cycle "input is applied "to terminals 50. "The amplitude or the input is varied'untiha clearly discernablezreadingappears meter '31'and 'thisrea'ding noted. Then,

alflfiOD cycle .wave of say-one-l tenth amplitude is anfilie'dio input terminals 15B. .The .new setting toiwhich potentiometer 1.00 .must be adjustedto give the .same .ireading as .hefore may be marked vrlth-ainumeral 1D indipatingthat the. intermodulationrproductzis equal to' ten percent pfrthe ftundamental. EFurth'er, :calibration mai ks may obtained, for example, by successively changing the-amplitude :o'f the 3600 cycle .wave to A, 1720,

1%100 etc.,-- of -the amplitude of the 4000-cyc1e input ar-1d :readjusting "potentiometer 100 toagain give the same reading on-the VU meter :By markingeach of athe positions-thus -:obtained in percentagexan'd tor nae rnany other ratios as de sired the potentiometer I00 may becalibrated as closely -as necessary. v I lt -will lee notedcthat.w l'ien'etone from terminals isted intoan=amplifier on-system operatingappreeiahly :below its maximum levl, the readings obtained 011 &meter 3 7 :probably cannot 'be' equalized -on-= steps 4 and 1 5:01? the input levelea'djustment potentiometer 8 1. "This indicates ztheit the apparatus under test his eparating. at suchra low level that :the :interm'odulation .is far :below the objectionable p'oin-t. 'At l'high levels :it will be found -that the intermodulati'on :rises quite rapidly. It has been determined that for san overall system, intermodulation lvalues .ofr5 or less are quite good. The potentiometer 6I;isrpro-'- vided as an input level :control to adjust the amplitudeofitones :whichiare-applied toatermin'ais to a value such :asito: affordxa i.convenient"fon scale indication oi -the referenceton Correct level is indicated, when -:the :V.U meter 31 :on'position :4 .of the .switches 4:0, 'JH and :M, :indieates well up scale,ithatiis, abovmminus 1o Wand below plust2 VU.

it-he output appearing at :terminals 3 3;is satisfactory forifeeding :circuits having a characteristic impedance vlrangingifrom .100 .ohms :to tinflnity. The;input impedanceat terminals 51! HS substantially infinite, ithat is, fit .15 zbridging.

:While .I ,havejllustrated :a :parti-eul'ar embodiment of the :present invention, it should be clearly understood that sit is :not limited thereto since "many :modifications :may .be made :in the several elements employed ;and in their arrange-. ment without departing from the: spirit; and scope of the invention. =Whatisclaimedis:

1. A distortion measuring'instrument;including a source-f ;high 'frequency wave energy; and a source of wave energy of lower lfrequeneygmeans' for; combining predetermined amounts of .energy fromaeach-of said sources,;means-;for applyingsaid-combined energies :to the input of adeviee under test, :means adapted to -:be coupled to theioutput of;saidadevice andresponsive only'to Wave .energypirequencies :in aynarrow frequency range adjacent to and .onboth sides of said high frequency source, means for determining the amplitude ofwave energy :at the output of said wave responsive means, :a filter :network coupled tolsaid-wave responsive means for: discriminating against-wave: energy of frequencies ina narrower frequency :range adjacent "to -and including :the frequency of said high .frequency source and means for :determining the ratio :of :wav energy at the output-of said filter -:to the wavev energyat the :output of said wave;=responsive means.

:2. ,A :distortion measuring instrument including a source of high frequency wave'energy and a.iSOl1ICB..'.Of :wave energy 'of lower frequency, means .ior :combining predetermined amounts of energy from :each of said sources, means, foriap plyingsaideombined energies to the input ofaa device under test, means adapted to be coupled to the output of-said device andresponsive only to waveenerg irequencies ina narrowirequency mnge adjacent to and on bothsidesof .saidfhih frequency source, means for determining the amplitude of wave energy at theroutput of said wave responsive means, a filter network coupled. to said wave responsive meansfor discriminating against. wave energy of frequencies in a narrower frequencyrange adjacent to and including the frequency of said high frequencysource, and means for selectively connecting a measuring instrument to the input and to the output of filter network.

3. An intermodulation meter including ahigh frequency and a low frequency tone generator, a circuit coupled to said generators and adapted to combinepredetermined amounts of eachof said" tones, connections for applying said combined tones to input terminals of a device under. test, a first filter arranged to discriminate against said low frequency tone and adapted to be coupled to output terminals of said device, connections for coupling a meter to the'output of said first filter, a second filter adapted to discriminate against said high frequency tone and against tones of frequencies closely adjacent said high frequency tone, the input of said second filter being coupled to the output of said first filter, and further connections for coupling said meter to'the output of said second filter.

:4. An intermodulation meter including a high frequency and a low frequency tone generator, acircuit coupled to said generators and adapted to combine predetermined amounts of each of said tones, connections for applying said combined tones to input terminals of a device under test, a first filter arranged to discriminate against said low frequency tone and adapted to be coupled to output terminals. of said device, connections for coupling a meter to the output of said first filter, a second filter adapted to discriminate against said high frequency and against tones of frequencies closely adjacent said high frequency tone, said second filter also being coupled to said first filter and further connections for coupling said meter to the output of said second filter, said connections being. selectively effective whereby the relative outputs of said;

two filters may be compared.-

5. An-intermodulation meter including a high frequency and a low frequency tone generator, a circuit coupled to said generators and adapted to combine predetermined amounts of each of said tones, connections for applying said combined tones to input terminals of a device under test,

a first filter arranged to discriminate against said" low frequency tone and adapted to be coupled to output terminals Of said device, con-.

' adjusting means in said second filter whereby said meter may be adjusted for equal readings in each of said selected connections, said amplitude adjusting means being calibrated in terms of decimal ratios of the output at closely adjacent frequencies to total output of said first filter. a q a 6. The'method of determining distortion-pro duced in an amplifier includingcombining pre-"l determined amplitudes of a high frequency and .said' the ratioof the said two measurements; l

a low frequency tone, applying said combined, tones to, the input of said amplifier, measuring theamplitude of all tones in a narrow frequency range adjacent to and on both sides of, said-high frequency tone in the output of said amplifier, measuring the amplitudeof tones in a-still nare rower frequency range immediately adjacent said high frequency. tone, excluding said highifree'. quency tone and tones of said narrower fre-, quency range, and determining the ratio of. the said two measurements. I 7. The method of determining distortion pro-e duced in an audio frequency.transducersystem including combining predetermined amplitudes.

; of a high frequency and a low frequencytone;

applying said combined tones to the input of said system, measuring the amplitude of'all tones in a narrow frequency range adjacent to and on both sides of said high frequency ton 'in' the output of said system, measuring the amplitude of tones in a still narrower frequency range immediately adjacent said high "frequency tone; excluding said high frequency tone andtones ofsaid narrower frequency range, and determining 8. The method of determining distortion pro duced in an amplifier including combining a high frequency and a low frequency tone in a four to one ratio, applying said combined tones tothe input of said amplifier, measuring the amplitude of all tones in a narrow frequency range adja cent to and onboth sides of said high frequency tone in the output of said amplifier, measuring the amplitude of tones in a still narrower frequency range immediately adjacent said high frequency tone, excluding said high frequency tone and'tones of said narrowerfrequency-ra'nge' and determining the ratio of the sai'd'two measurements. 7 Y 9. The method of determining distortion pro duced in an audio frequency transducer system including combining a high frequency and alow frequency tone in a four to one ratio,-applying'- said combined tones to the input of said transducer, measuring theamplitude of all tones in a narrow frequency range adjacent to and on both sides of said high frequency tone in the output of said transducer, measuring the amplitude of tones in a still narrower frequency range immediately adjacent said high frequency tone, in: cluding said high frequency tone and tones of said narrower frequency range and determining the ratio of the said two measurements.

10. The method of determining distortion" pro-=1 duced in an amplifier including combining ahigh frequency and a low frequency tone in a four, to one ratio, applying said combined tones to the input of said amplifier, selecting from the output of said amplifier all tones in a narrow frequency range adjacent to and onboth side of said high frequency tone, measuring the combined ampli{ tude of said tones, discriminating against said high frequency tone, measuring the amplitude of the remainder and determining the ratioof.

* said two measurements.

said combined tones to the input of said trans-"1 ducer, selecting from the output of said trans-j ducer all tones in a narrow frequency rangefadj aa J cent to and on both sides of said high frequency tone, measuring the combined amplitude'of said tones, discriminating against said high frequency 9. tone and tones of a still narrower frequency range immediately adjacent said high frequency tone, measuring the amplitude of the remainder and determining the ratio of said two measurements.

12. An intermodulation meter including a high frequency and a low frequency ton generator, an amplifier having an input circuit coupled to both of said generators, said coupling being variable so that the relative amounts of said tones applied to said input may be adjusted, the output circuit of the amplifier being adapted to be coupled to the input circuit of a device to be tested, a VU meter adapted to be coupled to said output circuit, means for selectively disabling first one and then the other of said tone generators, a voltage divider network across said output circuit having a pair of taps thereon so disposed that the voltages thereacross are in a ratio of four to one, means for connecting the VU meter to the higher of said taps when said high frequency generator is disabled and the other when said low frequency generator is disabled, a filter network adapted to be coupled to the output of said device and adapted to discriminate against low frequency tones, a second filter and amplifier network coupled to said first filter, said second filter being adapted to discriminate against tones of frequencies equal to and closely adjacent said high frequency tones, switching means for selectively connecting said meter to the input and to the output of said second filter, a potential divider in said second filter and means for so adjusting said divider as to obtain equal readings on said meter from the input and output of said second filter.

13. An intermodulation meter including a high frequency and a low frequency tone generator, an amplifier having an input circuit coupled to both of said generators, said coupling being variable so that the relative amounts of said tones applied to said input may be adjusted, the output circuit of the amplifier being adapted to be coupled to the input circuit of a device to be tested, a filter network adapted to be coupled to th output of said device and adapted to discriminate against low frequency tones, a second filter and amplifier network coupled to said first filter, said second filter being adapted to discriminate against tones of frequencies equal to and closely adjacent said high frequency tone, switching means for selectively connecting a VU meter to the input and to the output of said second filter, a potential divider in said second filter and means for so adjusting said divider as to obtain equal readings on said meter from the input and output of said second filter.

JARRETT L. HATHAWAY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,143,094 Swift Jan. 10, 1939 2,213,099 Adorjan Aug. 2'7, 1940 2,385,186 Bayless Sept. 18, 1945 

